1. Field of the Invention
The present invention relates generally to sandwich panels and other related structural composite materials. Sandwich panels are typically made up of face sheets (also referred to as “skins”) that are adhesively bonded to opposite sides of a core material, such as honeycomb, to form the sandwich panel. In particular, the present invention relates to treating honeycomb in order to improve the bond between the honeycomb and face sheets. The invention is particularly applicable to sandwich panels where the face sheets are made from composite materials.
2. Description of Related Art
Sandwich panels are used in a wide variety of applications where high strength and light weight are required. The cores that are used in most sandwich panels are either lightweight honeycomb, rigid foam, paper or wood. Honeycomb is a popular core material because of its high strength to weight ratio and resistance to fatigue failures. A wide variety of products including metals and composite materials have been used to make the honeycomb cores.
The face sheets that are bonded to the side (also referred to as the “edge”) of the honeycomb have also been made from a wide variety of materials including metals and composites. An important consideration in the formation of any sandwich panel is the way in which the face sheets are bonded to the honeycomb. Typically, an adhesive is used to bond the face sheets to the core. The adhesive must rigidly attach the facings or skins to the core in order for loads to be transmitted from one facing to the other and to permit the structure to fulfill all the assumptions implied in the acceptance of the commonly used stress calculation methods. If the adhesive fails, the strength of the panel is severely compromised. The adhesive is especially critical in sandwich panels which use honeycomb as the core because of the relatively small surface area over which the edges of the honeycomb contact the face sheets.
One procedure for applying composite face sheets to honeycomb involves forming a prepreg sheet that includes at least one fibrous reinforcement layer and an uncured resin matrix. Prepreg is a term of art used in the composite materials industry to identify mat, fabric, non-woven material, tow or roving which has been pre-impregnated with resin and which is ready for final curing. A film adhesive is typically added to the prepreg-core assembly and it is then bonded to the honeycomb by curing of both the prepreg resin and adhesive resin at elevated temperature. The film adhesive can be applied as a separate ply layer or as an integral part of the prepreg sheet.
Honeycomb sandwich panels are used in many applications where stiffness and structural strength of the panel are primary considerations. Additionally, honeycomb sandwich panels are also widely used in the aerospace industry where the weight of the panel is also of primary importance. As a result, there has been and continues to be a concerted effort to reduce the weight of the honeycomb sandwich panels without sacrificing structural strength. One area which has been investigated to reduce weight is the elimination of separate adhesive layers. This has been accomplished by making the face sheets from composite materials that are self-adhesive. The resins used in such self-adhesive prepregs must meet the dual requirements of providing suitable structural strength while still providing adequate adhesion to the honeycomb. Exemplary self-adhesive face sheets are described in published European Patent Application Nos. EP0927737 A1 and EP0819723 A1 and in U.S. Pat. Nos. 6,440,257 and 6,508,910.
An alternative method of bonding face sheets to honeycomb involves applying an adhesive to the edge of the honeycomb. The adhesive is typically applied by “dipping” the edge of the honeycomb in the adhesive. The adhesives used in this type bonding are typically referred to as “dip” resins or adhesives. The advantage of this method is that the adhesive is located only where the honeycomb contacts the face sheet, rather than being distributed over the entire face sheet. This method is generally used to bond non-adhesive face sheets, such as aluminum and other metallic face sheets, to the honeycomb.